The present invention relates to an optical recording medium for recording, reproducing and erasing information by using a laser beam and to a recording method of the same.
Conventionally, in the case that information is recorded on an optical disk, an intensity of laser light is modulated to record a circular or elliptical recording mark. In the pit-position recording method whereby information "1" is associated with the central position of this recording mark, the laser light intensity is increased up to a predetermined power only when a circular recording mark is recorded. For a portion in which information is not recorded, i.e., for a portion of information "0", the laser light is not radiated or intensity of laser light is lowered up to such a degree that a servo such as auto focus or tracking is effected. In the pit-edge recording method whereby information "1" is associated with both edges of a recording mark, the laser light intensity is increased up to a predetermined power only when the elliptical marks of various lengths are recorded. In optical magnetic disks and optical disks of phase change type, the light intensity is increased up to a predetermined power when a mark is recorded, whereas the intensity of laser light is lowered up to power lying midway between the recording power and servo power when a recording mark is erased, i.e., information "0" is recorded.
In any of the recording methods described above, only circular or elliptical recording marks can be recorded so long as a conventional light intensity modulation method is used. As the recording marks become smaller, therefore, the signal to noise ratio significantly deteriorates, higher recording density being hindered. That is to say, in the case that the density in the rotation direction (length direction) of the disk is being increased, shortening the length of a mark simultaneously shortens the width of the mark as well. The magnitude of a signal read out at a light spot is nearly in proportion to the area of the recording mark. If the length of a circular recording mark is halved, for example, therefore, its width is also halved and the signal output is reduced to a quarter. Furthermore, in case of recording using the light intensity modulation, heat generated by the light pulse radiation propagates in the travelling direction of the light spot and affects mark formation made by the next light pulse. Due to such thermal interference, it is difficult to record minute marks consecutively.
As an example of a known technique relating to an optical magnetic recording method allowing the light intensity modulation overwriting operation, JP-A-3-156751, for example, can be mentioned. In this method, an optical magnetic recording medium having an exchange coupling four-layered film is used and direct overwriting is conducted by using the light intensity modulation. According to this method, a ring-shaped recording mark is formed by the self-sharpening effect. However, this is recorded in such a form that the area surrounding the recording mark is erased. Thus a method of actively recording a specific ring-shaped recording mark is not disclosed in JP-A-3-156751.
On the other hand, the recording marks other than circular marks are formed in the magnetic field modulation method for optical magnetic disks. In this magnetic field modulation method, the laser light having such a constant intensity that recording can be conducted is kept radiated and only the magnetic field applied from the outside is modulated to overwrite information. The marks formed at this time are crescent-shaped as described in JP-B-3-41906, for example. Even if the length (track direction) of recording marks is changed in this case, the width (radius direction) does not change. Even if the length of recording marks is halved, therefore, the signal is only lowered to a half. Furthermore, since DC light is radiated, the temperature distribution becomes always constant and the thermal interference described above is absent. Therefore, the magnetic field modulation method is suitable for high density recording.
In this magnetic field modulation method, however, it is very difficult to modulate the magnetic field applied from the outside in the order of several tens of megahertz. If it is dared to be executed, a magnetic field generating device of contact type such as a magnetic head becomes necessary. Thus, there is a problem that interchangeability featuring optical magnetic disks is lost.
In the conventional light intensity modulation method allowing overwriting operation, only circular or elliptical marks can be recorded. As the marks become smaller, therefore, the signal to noise ratio is significantly degraded, the higher recording density being thus hindered. Furthermore, it is difficult to record minute recording marks consecutively under the influence of thermal interference of laser light spots. Furthermore, in the magnetic field modulation method whereby a mark having width larger than its length can be recorded without being affected by thermal interference, it is difficult to attain higher density because of the problem of magnetic field switching speed.